The present invention generally relates to a receiver, a transmitter and a radio communication system, and a channel estimation method for a MIMO (Multiple Input Multiple Output) system in which both the receiver and the transmitter are provided with a plurality of antennas.
In conventional mobile communication systems, channel estimation generally has been performed using pilot signals, and channel compensation and equalization have been employed for detecting information symbols. In order to increase the channel estimation accuracy, more pilot signals are required. However, if more pilot signals are employed in frames, the actual transmittable information in frames becomes less. As a solution to this problem, an iterative channel estimation method is known, in which detected information bits are utilized for estimating channels.
A receiver performing such an iterative channel estimation is explained with reference to FIG. 1.
A receiver 10 performing such an iterative channel estimation comprises a plurality (M) of antennas 1, a plurality of channel estimators 2 (2-1˜2-M) and a plurality of channel updating (or renewing) units 4 (4-1˜4-M) connected to the antennas 1, an information signal detector 3 connected to the antennas 1, a transmission symbol ‘s’ generator 9 receiving an output from the information signal detector and connected to the channel updating unit 4 (4-1˜4-M), and a controller 5 connected to the information signal detector 3 and ‘s’ generator 9. The information signal detector 3 is switch-ably connected to the channel estimators 2 (2-1˜2-M) or the channel updating units 4 (4-1˜4-M) via a switch 6. Each of the channel estimators 2 (2-1˜2-M) receives a pilot signal.
In operation, the antennas 1 receive signals and supply the received signals to the corresponding channel estimators 2 (2-1˜2-M). Each of the channel estimators 2 uses the received signal and pilot signals included therein to estimate channels, and inputs the channel estimation value to the information signal detector 3.
The information signal detector 3 uses the input channel estimation values and the received signals to detect information signals. The detected information signal such as information bits are output and supplied to the ‘s’ generator 9 also. The ‘s’ generator 9 uses the input information bits and performs a process the same as in the transmitter to generate estimated transmit symbols.
The estimated transmit symbols are input to each channel updating unit 4 (4-1˜4-M). Each channel updating unit 4 (4-1˜4-M) uses the input estimated transmit symbols instead of pilot signals to estimate channels. The channel estimators 2 (2-1˜2-M), the information signal detector 3, the channel updating units 4 (4-1˜4-M) and the ‘s’ generator 9 are controlled by the controller 5.
In this manner, channel estimation can be done using many received signals, and therefore the channel estimation accuracy can be improved. The channel estimation values can be used for detecting information in the information signal detector 3, and therefore more highly accurate detection is obtained.
On the other hand, there exists a MIMO channel signal transmission system that can realize high frequency usage efficiency. In the MIMO channel signal transmission system, both transmitter and receiver use a plurality of antennas and have a plurality of channels between the transmitter and receiver to obtain parallel transmission and diversity advantages. One problem with this MIMO channel signal transmission system is that there are many channels to be estimated and therefore many pilot signals are required.
A scheme of combining the MIMO channel signal transmission system and the iterative channel estimation system is known as shown in Japanese Patent Laid-Open Application No. 2003-152603. A receiver according to this scheme is explained with reference to FIG. 2, in which a transmitter simultaneously sends N different information data streams over the same frequency.
The receiver 10 comprises M antennas 1, a plurality of channel estimators 2 (2-1˜2-M) connected to the corresponding antennas 1, a receiving unit 7 connected to the antennas 1, and s1 generator 8-1˜sN generator 8-N connected to the corresponding channel estimators 2 (2-1˜2-M).
The receiver 10 have M channel estimators 30 for M antennas and each of the M channel estimators has to estimate N values for N transmission streams.
The channel estimators 2 (2-1˜2-M) use received data and pilot signals included in the received signals, estimate channels and input the channel estimation values to the receiving unit 7.
The receiving unit 7 uses the input channel estimation values and the received signals, detects information signals, and outputs the detected information signals such as information bits (st1, . . . , stN). The receiving unit 7 supplies the information bits (st1, . . . , stN) to the s1 generator 8-1, . . . , the sN generator 8-N.
Each of the s1 generator 8-1, . . . , the sN generator 8-N generates an estimated transmit symbol s1, . . . , sN from the input information bits, and inputs the generated estimated transmit symbols s1, . . . , sN to the corresponding channel estimator 2-1, . . . , 2-M.
The channel estimators 2-1, . . . , 2-M use the input estimated transmit symbols instead of the pilot signals to update (or renew) channel estimates. The channel estimators 2-1˜2-M, the receiving unit 7 and s generator 8-1˜the sN generator 8-N are controlled by a controller (not shown).
The structure of the channel estimator 2-1 in the receiver 10 is explained with reference to FIG. 3. Other channels estimators 2-2˜2-M are the same as the channel estimator 2-1 and therefore their explanations are omitted.
The channel estimator 2-1 comprises a channel estimator 2-11 receiving the received signal r1(t) and the pilot signal, a channel updating (or renewing) unit 2-12 connected to the channel estimator 2-11 and receiving the received signal r1(t) and the estimated transmit symbols (s1, . . . , sN), and a multiplexer 2-15 switchably connected to the channel estimator 2-11 or the channel updating unit 2-12 via switches 2-13. The channel estimator 2-1 further comprises a controller 2-14 connected to the channel estimator 2-11, the channel updating unit 2-12 and the switches 2-13.
In operation, the channel estimator 2-11 uses the received signal r1(t) and the pilot signal to estimate channels. Regarding the pilot signal, channel estimation can be comparatively easily done by making the pilot signals orthogonal among the streams. For example, it is possible to use a frame structure and channel estimation method disclosed in the following document.
“Turbo receiver with sc/simplified-MMSE (S-MMSE) type equalizer for MIMO channel signal transmission”, H. Fujii et. al., IEEE vtc 2003-Fall
On the other hand, during the data period, the received signals include a plurality of stream signals having no orthogonal relations. Then it is required to suppress interference between streams and estimate each channel.
The initial channel estimation values (h11, . . . , H1N) estimated in the channel estimator 2-11 are input to the channel updating unit 2-12 and the multiplexer 2-15 via the switch 2-13.
The multiplexer 2-15 multiplexes the input initial channel estimators and outputs.
The channel updating unit 2-12, based on the input initial channel estimation values and estimated transmit symbols (s1, . . . , sN), estimates channels, and supplies the channels estimation values to the multiplexer 2-15. The multiplexer 2-15 multiplexes the input channel estimation values and outputs.
The structure of the channel updating unit 2-12 is explained with reference to FIG. 4.
The channel updating unit 2-12 comprises a correlation vector calculator 2-121 receiving the received signals r1(t) and the estimated transmit symbols s1(t)˜sN(t), a correlation matrix calculator 2-122 receiving the estimated transmit symbols s1(t)˜sN(t), and a multiplier 2-123 connected to the correlation vector calculator 2-121 and the correlation matrix calculator 2-122.
The channel from the transmit antenna n to the receiving antenna m is represented by hmn, and the vector Hm is represented by Hm=[hm1 hm2 . . . hmN]T, the estimated transmit symbol is represented by sn(t), that is the vector S(t) is represented by S(t)=[(s1(t) s2(t) . . . sN(t)]T, and the received signal is represented by rm(t).
The correlation vector calculator 2-121 calculates a correlation vector Rxd by an equation Rxd=Σ(rm*(t)S(t))/Nsmp, where
Nsmp means the number of received signals used for the channel estimation.
The correlation matrix calculator 2-122 calculates a correlation matrix Rxx by an equation Rxx=Σ(S(t)S(t)H)/Nsmp, where
H means conjugated transpose.
The correlation vector Rxd calculated by the correlation vector calculator 2-121 and the correlation matrix Rxx calculated by the correlation matrix calculator 2-122 are input to the multiplier 2-123. The multiplier 2-123 obtains the channel Hn by an equation Hm=Rxx−1Rxd.
However, the above explained related art examples have the following problems.
In the channel estimation method using MMSE (Minimum Mean Square Error), it is required to use the degree of freedom of the filter to suppress other streams, and therefore channel estimation accuracy is degraded especially when the number of the received signals is few.
When using the MMSE, it is required to obtain an inverse matrix of training signals, and therefore the amount of calculations becomes large. Even if a RLS (Recursive Least Square) algorithm is used to converge the channel estimation values, the amount of calculations is still large.    [Patent Document #1]
Japanese Laid-open 2003-152603    [Patent Document #2]
“Turbo receiver with SC/Simplified-MMSE (S-MMSE) type equalizer for MIMO channel signal transmission”, H. Fujii et. al, IEEE VTC2003-Fall